Ink jet receiving materials used at the present time have a particular need for improvement in physical and handling properties, particularly in light fastness and water fastness together with an improved image quality. A preferred embodiment of this invention is therefore directed towards ink jet recording materials with improved handling and performance characteristics. In particular ink receiving materials are sought where the images recorded thereon are resistant to rubbing of the surface or to damage by other physical means, Moreover printed images should remain intact when in contact with water and particularly should not fade when exposed to light even under adverse conditions. The present invention provides a solution towards these problems.
Ink jet printing systems generally are of two types: continuous stream and drop on demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to brake up into droplets at a fixed distance from the orifice. At the break up point, the droplets are charged in accordance with digital data signals and passed trough an electric static field which adjusts the trajectory of each droplet in order to direct it to a specific location on a recording medium or a gutter for recirculation. In drop on demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
This invention is directed towards both methods, the more demanding continuous stream system as well as towards the more common drop on demand printing devices.
It is known that images created with a state of the art ink jet printer must meet a number of stringent demands. The following requirements are some of the features recording materials used in ink jet printing systems must necessarily fulfil in order to yield printed images of high quality:                1. Sufficient ink absorbing capacity and ink receptivity of the receiving layer to prevent the ink from streaking and from running down during printing, even under conditions where several droplets are deposited in a rapid sequence onto the same spot.        2. Fast drying of the layer surface after printing of the image leading to prints free from tackiness.        3. Excellent colour rendition, no change of hue of the picture with time.        4. Surface with high gloss.        5. In the case of transparencies, the printed images must be clear, transparent and scatter free.        6. Resistance of the image surface to rubbing.        7. Excellent water fastness of the printed images.        8. Excellent light fastness of the printed images.        9. Excellent archival stability of the printed images.        10. Excellent physical and handling properties before and after printing.        
The particular problem of water fastness has in the past been addressed by a wide variety of techniques. Thus solutions to the problem have been proposed by using specific formulations of the inks or alternatively in many cases by specific modifications of the receiving layers. The two approaches have occasionally been combined.
Solutions aimed at resolving the problems of ink absorbing capacity, drying time and image quality have in many instances been attempted by specific modification of the composition of the coated receiving systems. A widely used approach to control absorbtivity and to a certain extent dot spread is through incorporation of high molecular weight hydrophilic polymeric substances.
Particularly favoured in this respect are polyvinyl alcohol and polyvinyl pyrrolidone. U.S. Pat. No. 4,503,111 describes ink receiving layers where for instance gelatine, polyvinyl alcohol and polyvinyl pyrrolidone are mixed. This combination is claimed to allow the printing of large colour filled areas with high colour density without generating puddling and ink running. U.S. Pat. No. 4,592,951 describes layers of crosslinked polyvinyl alcohol aimed at solving the same problems. Additional examples of patents claiming solutions to the above mentioned or similar problems involving layers containing polyvinyl alcohol alone or together with additional hydrophilic polymers are U.S. Pat. Nos. 4,741,969, 4,892,787, 5,132,146 and 5,352,503 as well as EP Patent Applications 0,631,880 and 0,650,850. Most attempts to achieve high water fastness in receiving layers have been through the use of cationic polymers in conjunction with inks containing acidic dyes. Polyvinyl alcohol and cationic compounds are often used simultaneously. This approach has been described extensively in the prior art. The following are some selected typical examples: U.S. Pat. No. 4,877,680 describes cationic polymers together with neutral binders. Cationically modified polyvinyl alcohol has been described in U.S. Pat. No. 4,783,376. U.S. Pat. No. 4,575,465 claims the use of quaternised polyvinyl pyridine to achieve water fastness. U.S. Pat. No. 4,554,181 describes the use of a combination of cationic polymers and polyvalent metal salts since only such combinations and not the single elements are claimed to provide the sought after properties. U.S. Pat. No. 5,180,624 and EP Patent Application 0,634,289 again combine hydrophilic binders like for instance gelatine or polyvinyl alcohol with cationic substances like for instance quaternised polyvinyl pyridine or poly(meth)acryl quaternary salts for high optical density and uniform image definition. Further patents describing similar approaches are U.S. Pat. Nos. 4,801,497 and 5,270,103.
EP Patent Application 0,423,829 describes the combination of silica particles with polyvinyl alcohol and cationic water soluble acrylic polymers. Layers with good ink absorption and good image quality were obtained. The light fastness of the printed image does however not seem to satisfy today's demands. A similar approach using colloidal silica and a modified polyvinyl amine is described in WO Patent Application 94/26,530. Similarly EP Patent Application 0,423,829 claims polyvinyl alcohol and a cationic acrylic copolymer carrying at least two cationic radicals in the side chain as a solution to achieve water fastness of the printed image.
In JP Patent Application 01-008,085 cationic copolymers containing more than 30% amine groups were coated onto wood free paper base together with polyvinyl alcohol and finely powdered silica. The layers were subsequently heated to 120° C. The image is claimed to have good water fastness and light fastness. A similar approach has been taken in JP Patent Application 04-263,984. Polyvinyl alcohol groups taken together with primary or secondary amines or the corresponding ammonium salts are coated together with dialdehydes. This approach is less suitable for the preparation of permanent images due to the tendency of prints on such layers to yellow on ageing. EP Patent Application 0,631,881 describes mixtures of polyvinyl alcohol, polyvinyl pyrrolidone, vinyl acetate and poly (dimethyl-diallyl) cationic polymers aimed at achieving good water fastness.
A further approach aimed at improving ink receiving materials has been the use of vinyl based copolymers. U.S. Pat. No. 4,547,405 describes the preparation of receiving sheets containing a coalesced block copolymer latex of polyvinyl alcohol with polyvinyl (benzyl ammonium) chloride. U.S. Pat. No. 4,904,519 claims the preparation of an ink receptive layer comprising hydrolysed copolymers formed from 70 to 30% of vinyl amide monomers and vinyl ester monomers. Crosslinking is achieved with borates or other similarly acting salts and with aldehydes. The layers are claimed to accept ink readily and to be resistant to fingerprinting. The degree of hydrolysis has, as described in the patent, to be kept under tight control. If the amide part of the copolymer is even partially hydrolysed the resulting layers tend to remain tacky after printing. A similar approach has been taken in U.S. Pat. No. 4,944,988. Copolymers prepared from for instance vinyl acetate together with cationic tertiary amino radical containing acrylic monomers are used to prepare receiving layers without hydrolysing the esters. Polymers of this kind, although described to be beneficial towards improving image quality, have unfortunately due to their low basicity only low activity as mordants for inks based on anionic dyes. They do therefore influence only marginally the water fastness of the printed images.
U.S. Pat. No. 4,956,230 describes a blend of hydrophilic and hydrophobic polymers or copolymers free of OH, NH and NH2 groups. The major advantages of such polymers or copolymers are claimed to be good image quality, resistance to layer softening at high humidity and fast drying. No appreciable mordant activity can however be expected from such compounds.
Although improved water fastness can in many instances be obtained with a variety of such cationic elements, these systems on the other hand tend to show a severe drawback in that they seriously impair the light fastness of the printed images. Moreover such systems often tend to show poor water fastness despite the fact that such cationic polymers bind anionic or acid dyes quite efficiently This is often due to the fact that the layers as a whole disintegrate in water. This may also be due to poor adhesion of the layers to the support. This is particularly the case with photolike bases like polyolefin coated papers or polyester films.
Cationic polymeric substances have also been combined with metal salts, for instance in JP Patent Application 4,201,594 and in GB Patent 2,147,003. The major benefit of adding such metal salts seems to be an improved light fastness of the produced images.
An additional problem observed in many cases is irregular imaging density due to poor ink receiving properties of the receiving sheets. This may occur for different reasons. Among others it may be due to the uncontrolled binding of the dyes on the surface due to a particular mordant system. Another reason may be the poor wetting of the layer surface by the ink due to a particular cationic polymer. Layers with rapid ink uptake seem to be a prerequisite for good image quality. EP Patent Application 0,445,327 describes receiving layers with good image quality on polyolefin coated paper. The receiving layers consist of a mixture of gelatine and starch powder. Excellent image quality is described for layers containing hydrophilic polymers or mixtures of polymers like gelatine with polyvinyl alcohol or polyvinyl pyrrolidone when used together with water based inks (Journal of Imaging Science 30, 4, 1986). Gelatine has also been combined with cationic polymers, in particular with quaternised polyvinyl pyridine, and has been claimed to give reasonably good images in U.S. Pat. No. 4,575,465. Quaternised pyridinium compounds are however notorious in inducing poor light fastness of the acidic dyes usually used in ink jet printing.
In practically all cases known in the art where water fastness is desired it is achieved by making use of the mordanting effect of cationic species based on tertiary amino radicals. Although good water fastness is achieved in many cases, improvement of light fastness in particular is in general neither observed nor claimed.
The ideal combination of mordants, binders and crosslinking agents leading to printed images combining good water fastness, image quality, archival stability together with excellent light fastness is still lacking.